1. Field of the Invention
The present invention relates to a solid-state laser apparatus which is capable of stably generating high-power laser beams having a satisfactory quality.
2. Description of the Related Art
FIG. 13 is a schematic view showing a conventional semiconductor laser exciting solid-state laser apparatus disclosed in, for example, PCT WO96/35246. Referring to the drawing, reference numeral 1 represents a semiconductor laser, 2 represents exciting light, 3 represents a solid-state laser medium containing an active solid medium, 4 represents a reflecting mirror, 5 represents a partial reflecting mirror, 6 represents a laser beam, 7 represents an optical resonator, 8 represents an optical fiber, 9 represents a lens for collimating exciting light and 11 represents a lens for converging exciting light. Referring to the drawing, symbol L represents a length of the resonator and L1 represents a distance from the reflecting mirror 4 to the solid-state laser medium 3. Symbol L2 represents a distance from the partial reflecting mirror 5 to the solid-state laser medium 3. Symbol F represents a focal distance of a thermal lens generated in the solid-state laser medium. The optical resonator 7 incorporates the reflecting mirror 4, the partial reflecting mirror 5, the solid-state laser medium 3 and the thermal lens F realized by dint of distribution of temperatures in the solid-state laser medium.
The conventional semiconductor laser exciting solid-state laser apparatus has the above-mentioned structure. Thus, exciting light 2 emitted from the semiconductor laser 1 is transmitted through the optical fiber 10, and then introduced into the solid-state laser medium 3 by dint of the lenses 8 and 9. Thus, the solid-state laser medium 3 is excited so as to be formed into a laser amplifying medium. Moreover, the thermal lens having a focal distance F is generated in the solid-state laser medium. Naturally emitted light generated by the laser amplifying medium is amplified during reciprocating motions between the reflecting mirror 4 and the partial reflecting mirror 5. Thus, a laser beam 6 having excellent directivity is generated. When the intensity of the laser beam 6 exceeds a predetermined level, the laser beam 6 is emitted to the outside from the partial reflecting mirror 5.
As disclosed in a document (Optical Electronics, 4th ed., Holt, Rinechart and Winston, Inc,. pp. 125-127), a formula showing a stabilizing condition for a resonator and using beam matrix elements A, B, C and D with respect to light which reciprocates one time in the resonator is represented as follows: EQU -1&lt;(A+D)/2&lt;1 (1)
A beam matrix required to represent the stabilizing condition for a resonator will now be described. Transmission and reflection characteristics of a beam for a various optical elements can be represented by a beam matrix which is a matrix composed of two rows and two columns. Elements of the beam matrix are represented by A, B, C and D. Assuming that a symmetrical axis of a cylindrical coordinates which is an optical axis of each optical element is axis z, a distance on a plane of incident on the optical element from an optical axis of a beam is rin, inclination of the beam drin/dz is rin', a distance on an emission place of the optical element from the optical axis of the beam is rout and inclination of the beam is drout/dz is rout', the foregoing factors can be related with one another by two equations which are rout=Arin+Brin' and rout'=Crin+Drin'. When column matrix r1={rin, rin'} and r2={rout, rout'} are used and the beam matrix is represented by M, the two equations can be expressed as r2=mr1 by using a rule of the product of matrices. The relationship between column matrix r1 of incident light and column matrix r2 of emission light in a case where light passes through, for example, two optical elements is made to be r2=M2M1r1 by arranging the beam matrices in accordance with the rule of the product of the matrices on the assumption that the beam matrix representing a first optical element is M1 and the beam matrix representing the second beam matrix is M2. Similarly, a column matrix of emission light in a case where light passes through a larger number of optical elements can be obtained by calculating the product of the beam matrix of each optical element and the column matrix of incident light in accordance with the rule of the product of matrices. A representative beam matrix representing an optical element will now be described. A beam matrix representing a free space having a refractive index of n0 and a length of L is {A, B, C, D}={1,L/n0, 0, 1). A beam matrix representing a thin lens having a refractive index of f is {A, B, C, D}={1, 0,-1/f, 1} if a case in which f&gt;0 is assumed as a condensing lens. A beam matrix representing a spherical reflecting mirror having a curvature radius of R is {A, B, C, D}={1, 0,-2/R, 1} if a case in which R&gt;0 is a concave reflecting mirror. A beam matrix representing a dielectric medium interface having a refractive index of an incident portion of n1, a refractive index of an emission portion of n2 and a curvature radius of R is {A, B, C, D}={1, 0, (n2-n1)/n2R, n1/n2} if a case in which R&gt;0 is a concave. The stabilizing condition for the optical resonator can be obtained from a condition that a light beam must be returned to an original state after it has reciprocated in the optical resonator in a stable mode. Assuming that elements of a beam matrix indicating one reciprocating motion in the optical resonator are A, B, C and D, the condition is -1&lt;(A+D)/2&lt;1.
A stabilizing condition for the resonator of the conventional semiconductor laser exciting solid-state laser apparatus shown in FIG. 13 can be obtained by assuming a curvature radius of the reflecting mirror 4 is R1 and a curvature radius of the partial reflecting mirror 5 is R2 and by adding one to the stabilizing equation and by dividing a result of the addition with two, the stabilizing condition being represented as follows: EQU 0&lt;(1-L2/F-(L1+L2-(L1L2/F))/R1)(1-L1/F-(L1+L2-(L1L2/F))/R2)&lt;1 (2)
Since the conventional semiconductor laser exciting solid-state laser apparatus has an arrangement with which L1&lt;&lt;L2, F=L, R1 and R2&gt;&gt;L are satisfied, the stabilizing condition for the resonator is satisfied.
The conventional solid-state laser apparatus has the structure that the optical resonator is made to be a stable resonator by the reflecting mirror, the partial reflecting mirror, the solid-state laser medium and the thermal lens generated in the solid-state laser medium by dint of exciting light. Therefore, if a wavelength converting device or a transmission optical device which generate a thermal lens exists in the optical resonator, the stabilizing condition for the resonator cannot be satisfied. Therefore, the conventional structure cannot stably and efficiently generate high-power laser beams.
The conventional solid-state laser apparatus is structured in consideration of only exciting light as a factor for generating a thermal lens. Therefore, if a thermal lens is generated in the wavelength-converting device or a transmission optical device in the resonator because of a factor other than exciting light, such as a laser beam, the stabilizing condition for a resonator cannot be satisfied. Therefore, the conventional structure cannot stably and efficiently generate high-power laser beams.
The conventional solid-state laser apparatus is structured in consideration of only exciting light as a factor for generating a thermal lens. Therefore, if a thermal lens is generated in the wavelength-converting device or a transmission optical device in the resonator because of a factor other than exciting light, such as a laser beam, an optical resonator which is able to satisfy the stabilizing condition for a resonator cannot be designed.